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ocean worlds’ evolution and habitability in the solar system
Giuseppe Mitri, Frank Postberg, Jason M. Soderblom, Peter Wurz, Paolo Tortora, Bernd Abel, Jason W. Barnes, Marco Berga, Nathalie Carrasco,
Athena Coustenis, et al.
To cite this version:
Giuseppe Mitri, Frank Postberg, Jason M. Soderblom, Peter Wurz, Paolo Tortora, et al.. Explorer of Enceladus and Titan (E2 T): Investigating ocean worlds’ evolution and habitability in the solar system. Planetary and Space Science, Elsevier, 2018, 155, pp.73-90. �10.1016/j.pss.2017.11.001�.
�insu-01636074�
Accepted Manuscript
Explorer of Enceladus and Titan (E2T): Investigating ocean worlds' evolution and habitability in the solar system
Giuseppe Mitri, Frank Postberg, Jason M. Soderblom, Peter Wurz, Paolo Tortora, Bernd Abel, Jason W. Barnes, Marco Berga, Nathalie Carrasco, Athena Coustenis, Jean Pierre Paul de Vera, Andrea D'Ottavio, Francesca Ferri, Alexander G. Hayes, Paul O. Hayne, Jon K. Hillier, Sascha Kempf, Jean-Pierre Lebreton, Ralph D. Lorenz, Andrea Martelli, Roberto Orosei, Anastassios E. Petropoulos, Kim Reh, Juergen Schmidt, Christophe Sotin, Ralf Srama, Gabriel Tobie, Audrey Vorburger, Véronique Vuitton, Andre Wong, Marco Zannoni
PII: S0032-0633(17)30144-7
DOI: 10.1016/j.pss.2017.11.001 Reference: PSS 4418
To appear in: Planetary and Space Science Received Date: 29 April 2017
Revised Date: 11 September 2017 Accepted Date: 1 November 2017
Please cite this article as: Mitri, G., Postberg, F., Soderblom, J.M., Wurz, P., Tortora, P., Abel, B., Barnes, J.W., Berga, M., Carrasco, N., Coustenis, A., Paul de Vera, J.P., D'Ottavio, A., Ferri, F., Hayes, A.G., Hayne, P.O., Hillier, J.K., Kempf, S., Lebreton, J.-P., Lorenz, R.D., Martelli, A., Orosei, R., Petropoulos, A.E., Reh, K., Schmidt, J., Sotin, C., Srama, R., Tobie, G., Vorburger, A., Vuitton, Vé., Wong, A., Zannoni, M., Explorer of Enceladus and Titan (E2T): Investigating ocean worlds' evolution and habitability in the solar system, Planetary and Space Science (2017), doi: 10.1016/
j.pss.2017.11.001.
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Explorer of Enceladus and Titan (E2T): Investigating Ocean Worlds' Evolution and 1
Habitability in the Solar System 2
3
Giuseppe Mitri 1, Frank Postberg 2, Jason M. Soderblom 3, Peter Wurz 4, Paolo Tortora 5, Bernd Abel 4
6, Jason W. Barnes 7, Marco Berga 8, Nathalie Carrasco 9, Athena Coustenis 10, Jean Pierre Paul de 5
Vera 11, Andrea D’Ottavio 8, Francesca Ferri 12, Alexander G. Hayes 13, Paul O. Hayne 14, Jon K.
6
Hillier 15, Sascha Kempf 16, Jean-Pierre Lebreton 17, Ralph D. Lorenz 18, Andrea Martelli 8, Roberto 7
Orosei 19, Anastassios E. Petropoulos 14, Kim Reh 14, Juergen Schmidt 20, Christophe Sotin 14, Ralf 8
Srama 21, Gabriel Tobie 1, Audrey Vorburger 4, Véronique Vuitton 22, Andre Wong 14, Marco Zannoni 9
10 5
1 LPG, Université de Nantes, France 11
2 Klaus-Tschira-Laboratory for Cosmochemistry, University of Heidelberg, Germany 12
3 Massachusetts Institute of Technology, USA 13
4 University of Bern, Switzerland 14
5 University of Bologna, Italy 15
6 University of Leipzig Germany 16
7 University of Idaho, USA 17
8 Thales Alenia Space, Italy 18
9 LATMOS, France 19
10 LESIA, Observ. Paris-Meudon, CNRS, Univ. P. et M. Curie, Univ. Paris-Diderot, France 20
11 DLR, Germany 21
12 University of Padova -CISAS, Italy 22
13 Cornell University, USA 23
14 Jet Propulsion Laboratory, California Institute of Technology, USA 24
15 University of Kent, UK 25
16 University of Colorado, USA 26
17 LPC2E, France 27
18 JHU Applied Physics Laboratory, USA 28
19 INAF, Italy 29
20 University of Oulu, Finland 30
21 University of Stuttgart, Germany 31
22 Univ. Grenoble Alpes, CNRS, IPAG, France 32
33
11 September 2017 34
35
Manuscript submitted to Planetary Space Science 36
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Corresponding Author:
38 39
Giuseppe Mitri 40
Laboratoire de Planetologie et de Geodynamique 41
Université de Nantes 42
2 rue de la Houssinière 43
44322 Nantes, France 44
Giuseppe.Mitri@univ-nantes.fr 45
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Abstract 46
Titan, with its organically rich and dynamic atmosphere and geology, and Enceladus, with its 47
active plume, both harbouring global subsurface oceans, are prime environments in which to 48
investigate the habitability of ocean worlds and the conditions for the emergence of life. We 49
present a space mission concept, the Explorer of Enceladus and Titan (E2T), which is 50
dedicated to investigating the evolution and habitability of these Saturnian satellites. E2T is 51
proposed as a medium-class mission led by ESA in collaboration with NASA in response to 52
ESA’s M5 Cosmic Vision Call. E2T proposes a focused payload that would provide in-situ 53
composition investigations and high-resolution imaging during multiple flybys of Enceladus 54
and Titan using a solar-electric powered spacecraft in orbit around Saturn. The E2T mission 55
would provide high-resolution mass spectrometry of the plume currently emanating from 56
Enceladus’ south polar terrain and of Titan’s changing upper atmosphere. In addition, high- 57
resolution infrared (IR) imaging would detail Titan’s geomorphology at 50–100 m resolution 58
and the temperature of the fractures on Enceladus’ south polar terrain at meter resolution.
59
These combined measurements of both Titan and Enceladus would enable the E2T mission 60
scenario to achieve two major scientific goals: 1) Study the origin and evolution of volatile- 61
rich ocean worlds; and 2) Explore the habitability and potential for life in ocean worlds. E2T’s 62
two high-resolution time-of-flight mass spectrometers would enable resolution of the 63
ambiguities in chemical analysis left by the NASA/ESA/ASI Cassini-Huygens mission 64
regarding the identification of low-mass organic species, detect high-mass organic species for 65
the first time, further constrain trace species such as the noble gases, and clarify the evolution 66
of solid and volatile species. The high-resolution IR camera would reveal the geology of 67
Titan’s surface and the energy dissipated by Enceladus’ fractured south polar terrain and 68
plume in detail unattainable by the Cassini mission.
69
Keywords: Enceladus; Titan; Origin of volatiles; Habitability 70
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1. Introduction 71
The NASA/ESA/ASI Cassini-Huygens mission has revealed Titan and Enceladus to be two 72
unique worlds in the Solar System during its thirteen years of observations in the Saturnian 73
system (July 2004 - September 2017). Titan, with its organically rich and dynamic 74
atmosphere and geology, and Enceladus, with its active plume system composed of multiple 75
jets (Waite et al., 2006; Spahn et al., 2006; Porco et al., 2006), both harbouring global 76
subsurface oceans (Iess et al., 2010, 2012, 2014; see also discussion in Sotin et al., 2010), are 77
ideal environments in which to investigate the conditions for the emergence of life and the 78
habitability of ocean worlds as well as the origin and evolution of complex planetary systems.
79
The prime criteria of habitability include energy sources, liquid water habitats, nutrients and a 80
liquid transport cycle to move nutrients and waste (McKay et al., 2008, 2016; Lammer et al., 81
2009). The best-known candidates in the Solar System for habitability at present meeting 82
these criteria are the ocean worlds in the outer Solar System, which include: Enceladus, Titan, 83
Europa, and Ganymede (Lunine, 2016; Nimmo and Pappalardo, 2016). While the Jovian 84
moons will be thoroughly investigated by the ESA Jupiter Icy moon Explorer (JUICE), 85
Enceladus and Titan, which provide environments that can be easily sampled from orbit in a 86
single mission, are currently not targeted by any future exploration.The joint exploration of 87
these two fascinating objects will allow us to better understand the origin of their organic-rich 88
environments and will give access to planetary processes that have long been thought unique 89
to the Earth.
90
Titan is an intriguing world that is similar to the Earth in many ways, with its dense 91
nitrogen-methane atmosphere and familiar geological features, including dunes, mountains, 92
seas, lakes and rivers (e.g., Stofan et al., 2007; Lorenz et al., 2006, 2009; Lopes et al., 2007a;
93
Mitri et al., 2010). Titan undergoes seasonal changes similar to Earth, driven by its orbital 94
inclination of 27° and Saturn’s approximately 30 year orbit. Exploring Titan then offers the 95
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possibility to study physical processes analogous to those shaping the Earth’s landscape, 96
where methane takes on water’s role of erosion and formation of a distinct geomorphological 97
surface structure.
98
Enceladus is an enigma; it is a tiny moon (252 km radius) that harbours a subsurface 99
liquid-water ocean (Iess et al., 2014; McKinnon et al., 2015; Thomas et al., 2016; Čadek et 100
al., 2016), which jets material into space. The eruption activity of Enceladus offers a unique 101
possibility to sample fresh material ejected from subsurface liquid water and understand how 102
exchanges with the interior controls surface activity, as well as to constrain the geochemistry 103
and astrobiological potential of internal oceans on ocean worlds (e.g., Porco et al., 2006).
104
Since the 1997 launch of the Cassini-Huygens mission, there has been great technological 105
advancement in instrumentation that would enable answering key questions that still remain 106
about the Saturnian ocean worlds.
107
The scientific appeal of Titan and Enceladus has stimulated many previous mission studies 108
(e.g. see reviews by Lorenz, 2000; 2009), which have articulated detailed scientific objectives 109
for post-Cassini scientific exploration (e.g. Mitri et al., 2014a; Tobie et al., 2014). At Titan, in 110
particular, the diversity of scientific disciplines (Dougherty et al., 2009) has prompted the 111
study of a variety of observing platforms from orbiters (“Titan Explorer”, Leary et al., 2007;
112
Mitri et al., 2014a), landers for the seas (“Titan-Saturn System Mission – TSSM”, Strange et 113
al., 2009; “Titan Mare Explorer - TiME”, Stofan et al., 2013, Mitri et al., 2014a), landers for 114
land (Titan Explorer), fixed-wing aircraft (“AVIATR”, Barnes et al., 2012), to balloons (Titan 115
Explorer, TSSM and others). Additionally, Enceladus’ plume has attracted designs of 116
spacecraft to sample it: “Titan and Enceladus Mission TANDEM” (Coustenis et al., 2009), 117
“Journey to Enceladus and Titan – JET” and “Enceladus Life Finder – ELF” (Reh et al., 118
2016).
119
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We present a space mission concept, the Explorer of Enceladus and Titan (E2T), which is 120
dedicated to investigating the evolution and habitability of these Saturnian satellites and is 121
proposed as a medium-class mission led by ESA in collaboration with NASA in response to 122
ESA’s M5 Cosmic Vision Call. In Section 2 we present the science case for the future 123
exploration of Enceladus and Titan as proposed by the E2T mission, and Section 3 the science 124
goals for the E2T mission. In Sections 4 and 5 we discuss the proposed payload and mission 125
and spacecraft configuration necessary to achieve E2T mission goals.
126 127
2. Science Case for the Exploration of Enceladus and Titan 128
Titan, Saturn’s largest satellite, is unique in the Solar System with its dense, extensive 129
atmosphere composed primarily of nitrogen (97%) and methane (1.4%) (e.g., Bézard, 2014), 130
and a long list of organic compounds resulting from multifaceted photochemistry that occurs 131
in the upper atmosphere down to the surface (e.g., Israël et al., 2005; Waite et al., 2007;
132
Gudipati et al., 2013; Bézard, 2014). As methane is close to its triple point on Titan, it gives 133
rise to a methane cycle analogous to the terrestrial hydrological cycle, characterized by cloud 134
activity, precipitation, river networks, lakes and seas covering a large fraction of the northern 135
terrain (Figure 1) (e.g., Tomasko et al., 2005; Stofan et al., 2007; Mitri et al., 2007; Lopes et 136
al., 2007a; Hayes et al., 2008).
137
FIGURE 1 138
With an environment that changes on a 29.5 year cycle, it is crucial to study Titan during 139
an entire orbital period. Cassini has investigated Titan over only two seasons: from Northern 140
winter solstice to summer solstice. While ground-based observations, have observed Titan in 141
other seasons, these data are not sufficient to address many of the outstanding questions.
142
Current measurements with Cassini/CIRS show that the chemical content of Titan’s 143
atmosphere has significant seasonal and latitudinal variability (Coustenis et al., 2013; 2016);
144
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future extended exploration of Titan is necessary to get a full picture of the variations within 145
this complex environment.
146
Titan is the only known planetary body, other than the Earth, with long-standing liquid on 147
its surface, albeit hydrocarbons instead of water; these lakes and seas are likely fed by a 148
combination of precipitation, surface runoff and subsurface alkanofers (hydrocarbon 149
equivalent of aquifers) in the icy crust (Hayes et al., 2008). The presence of radiogenic noble 150
gases in the atmosphere indicates some communication between the surface and the 151
subsurface and is suggestive of water-rock interactions and methane outgassing processes 152
(Tobie et al., 2012), possibly associated with cryovolcanic activity or other exchange 153
processes (Lopes et al., 2007b, 2016; Solomonidou et al., 2014, 2016). The detection of a 154
salty ocean at an estimated 50-80 km depth (Iess et al., 2012; Beghin et al., 2012; Mitri et al., 155
2014b) and the possible communication between this ocean and the organic-rich surface hint 156
at exciting astrobiological possibilities. While Cassini has provided tantalizing views of the 157
surface with its lakes and seas, dunes, equatorial mountains, impact craters and possible 158
cryovolcanos, the low spatial, spectral, and mass resolution of the Cassini scientific 159
instruments make it difficult to identify morphological features, to quantify geological 160
processes and relationships between different geological units and to monitor changes due to 161
geologic or atmospheric activity. Constraining the level of geological activity on Titan is 162
crucial to understanding its evolution and determining if this ocean world could support 163
abiotic/prebiotic activity.
164
Both Titan and Enceladus possess several, if not all, of the key ingredients necessary for 165
life: an energy source, liquid habitats, nutrients (organic compounds) and a liquid transport 166
cycle to move nutrients and waste (McKay et al., 2008, 2016). While sunlight is a minimal 167
source of energy for solid bodies in the outer Solar System, interior heat sources derived from 168
a rocky core or tidal forces produced by neighbouring satellites and planet can be quite 169
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significant. Most recently, the Cassini INMS has identified molecular hydrogen at the level of 170
0.4–1.4% in Enceladus’ plume (Waite et al., 2017) providing further evidence of water-rock 171
interactions. This suggests that methane formation from CO2 in Enceladus’ subsurface ocean 172
could occur in a similar fashion as it occurs on Earth, where extremophile microbes in 173
hydrothermal sea vents produce methane as a metabolic by-product (McKay et al., 2008).
174
Another compelling discovery is the complex large nitrogen-bearing organic molecules in 175
Titan’s upper atmosphere by Cassini (Waite et al., 2007; Coates et al., 2007). The low 176
resolution of the in-situ mass spectrometers on Cassini, however, precludes the determination 177
of the chemical composition of this complex organic matter. In situ exploration with more 178
advanced instruments is required to investigate the prebiotic potential of Titan.
179
The discovery in 2005 of a plume emanating from multiple jets in Enceladus’ south polar 180
terrain is one of the major highlights of the Cassini–Huygens mission (Figure 2) (Dougherty 181
et al., 2006; Porco et al., 2006; Spahn et al., 2006). Despite its small size (10 times smaller 182
than Titan), Enceladus is the most geologically active moon of the Saturnian system.
183
Although geyser-like plumes have been observed on Triton (Soderblom et al., 1990) and more 184
recently transient water vapour activity around Europa has been reported (Roth et al., 2014;
185
Sparks et al., 2016, 2017), Enceladus is the only satellite for which this activity is known to 186
be endogenic in nature. The jets, of which approximately one hundred have been identified 187
(Porco et al., 2014) form a huge plume of vapour and ice grains above Enceladus’ south polar 188
terrain and are associated with elevated heat flow along tectonic ridges, called ‘tiger stripes’.
189
Enceladus’ endogenic activity and gravity measurements indicate that it is a differentiated 190
body, providing clues to its formation and evolution (Iess et al., 2014). Sampling of the plume 191
by Cassini’s instruments revealed the presence of water vapour, ice grains rich in sodium and 192
potassium salts (Postberg et al., 2011). Organic materials were observed, both in the gas 193
(Waite et al., 2009) and in the ice grains (Postberg et al., 2008, 2015), and molecular 194
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hydrogen (Waite et al., 2017). The jet sources are connected to a subsurface salt-water 195
reservoir that is likely alkaline in nature and undergoing hydrothermal water-rock interactions 196
(Porco et al., 2006, 2014; Waite et al., 2006, 2009, 2017; Postberg et al., 2009, 2011; Hsu et 197
al., 2011, 2014; Glein et al., 2015). The putative exothermic water-rock interactions on 198
Enceladus could be further constrained by quantifying He and constraining the amount of H2
199
in the plume. Gravity, topography and libration measurements demonstrate the presence of a 200
global subsurface ocean (Iess et al., 2014; McKinnon et al., 2015; Thomas et al., 2016; Čadek 201
et al., 2016; Beuthe et al., 2016). The co-existence of organic compounds, salts, liquid water 202
and energy sources on this small moon appear to provide all of the necessary ingredients for 203
the emergence of life by chemoautotrophic pathways (McKay et al., 2008) – a generally held 204
model for the origin of life on Earth in deep sea vents.
205
FIGURE 2 206
Titan and Enceladus offer an opportunity to study analogous prebiotic processes that may 207
have led to the emergence of life on Earth. Retracing the processes that allowed the 208
emergence of life on Earth around 4 Ga is a difficult challenge since most traces of the 209
environmental conditions at that time have been erased. It is, therefore, crucial for 210
astrobiologists to find extraterrestrial planetary bodies with similarities to our planet, 211
providing a way to study some of the processes that occurred on the primitive Earth, when 212
prebiotic chemistry was active. The eruption activity of Enceladus offers a possibility to 213
sample fresh material emerging from subsurface liquid water and to understand how exchange 214
processes with the interior control surface activity. It provides us with an opportunity to study 215
phenomena in-situ that has been important in the past on Earth and throughout the outer Solar 216
System.
217 218 219
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3. Scientific Objectives and Investigations 220
The proposed E2T mission has two major goals and several science objectives that would be 221
pursued through Enceladus and Titan investigations. The first scientific goal of the E2T 222
mission on Enceladus would focus on the origin and evolution of volatile compounds in the 223
plume vapour and icy grains. On Titan the first scientific goal includes two objectives; the 224
first would focus on determining the history and extent of volatile exchange on Titan and the 225
second objective would aim to understand how Titan’s surface processes evolved. The second 226
scientific goal on Enceladus would examine the nature of hydrothermal activity and search for 227
evidence of abiotic/prebiotic processes. On Titan, the second scientific goal would aim to 228
discern to what level of complexity abiotic/prebiotic chemistry has evolved. The scientific 229
objectives and investigations of E2T are discussed in detail in the Sections 3.1 and 3.2; Table 230
1 details the scientific objectives, the scientific questions, and the measurements requirements 231
(payload, instrument parameters) to address the scientific goals of the E2T mission.
232
Enceladus and Titan’ investigations would be conducted using the E2T mission model 233
payload, which consists of three instruments: two time-of-flight mass spectrometers, the Ion 234
and Neutral Gas Mass Spectrometer (INMS) and the Enceladus Icy Jet Analyzer (ENIJA) 235
dust instrument; and a high-resolution infrared (IR) camera, the Titan Imaging and Geology, 236
Enceladus Reconnaissance (TIGER). The scientific payload will be described in Section 4.
237
TABLE 1 238
239
3.1 Origin and Evolution of Volatile-Rich Ocean Worlds in the Saturn System 240
3.1.1 Chemical Constraints on the Origin and Evolution of Titan and Enceladus 241
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The origin and evolution of Titan’s methane still needs to be constrained. It is a key open 242
question whether Titan’s methane is primordial likely due to water-rock interactions in 243
Titan’s interior during its accretionary phase (Atreya et al., 2006) or else is delivered to Titan 244
during its formation processes (Mousis et al., 2009) or by cometary impacts (Zahnle et al., 245
1992; Griffith and Zahnle, 1995). On Titan, the Huygens probe detected a small argon 246
abundance (36Ar) and a tentative amount of neon (22Ne) in its atmosphere (Niemann et al., 247
2005, 2010), but was unable to detect the corresponding abundance of xenon and krypton.
248
The presence of 22Ne (36Ar /22Ne~1) was unexpected, as neon is not expected to be present in 249
any significant amounts in protosolar ices (Niemann et al., 2005, 2010), and may indicate 250
water-rock interactions and outgassing processes (Tobie et al., 2012). The non-detection of 251
xenon and krypton supports the idea that Titan’s methane was generated by serpentinization 252
of primordial carbon monoxide and carbon dioxide delivered by volatile depleted 253
planetesimals originating from within Saturn’s subnebula (e.g., Atreya et al., 2006). To 254
support a primordial methane source, xenon and krypton both would have to be sequestered 255
from the atmosphere. While xenon is soluble in liquid hydrocarbon (solubility of 10–3 at 95 K) 256
and could potentially be sequestered into liquid reservoirs, argon and krypton cannot be 257
sequestered as soluble materials (Cordier et al., 2010). Therefore, the absence of measureable 258
atmospheric krypton requires either sequestration into non-liquid surface deposits, such as 259
clathrates (Mousis et al., 2011), or depletion in the noble gas concentration of the 260
planetesimals (Owen and Niemann, 2009). Unlike Cassini INMS, the E2T INMS has the mass 261
range and the sensitivity to accurately measure xenon. E2T would measure the abundance of 262
noble gases in the upper atmosphere of Titan to discriminate between crustal carbon 263
sequestration and carbon delivery via depleted planetesimals.
264
The longevity of methane in Titan’s atmosphere is still a mystery. The value of 12C/13C in 265
Titan’s atmosphere has been used to conclude that methane outgassed ~107 years ago (Yung 266
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et al., 1984), and is being lost via photolysis and atmospheric escape (Yelle et al., 2008). It is 267
an open question whether the current methane rich atmosphere is a unique event, in a steady 268
state where methane destruction and replenishment are in balance (Jennings et al., 2009), or is 269
a transient event and is in a non-steady state where methane is being actively depleted or 270
replenished. Indeed, the possibility that Titan did not always possess a methane rich 271
atmosphere seems to be supported by the fact that the amount of ethane on Titan’s surface 272
should be larger than the present inventory; though Wilson and Atreya (2009) contend that 273
missing surface deposits may simply be reburied into Titan’s crust and Mousis and Schmitt 274
(2008) have shown that it is possible for liquid ethane to react with a water-ice and methane- 275
clathrate crust to create ethane clathrates and release methane. Nixon et al. (2012), however, 276
favours a model in which methane is not being replenished and suggest atmospheric methane 277
duration is likely between 300 and 600 Ma given that Hörst et al. (2008) demonstrated that 278
300 Ma is necessary to create Titan’s current CO inventory and recent surface age estimates 279
based on cratering (Neish and Lorenz, 2012). Mandt et al. (2012) suggests that methane’s 280
presence in the atmosphere, assumed here to be due to outgassing, has an upper limit of 470 281
Ma or else up to 940 Ma if the presumed methane outgassing rate was large enough to 282
overcome 12C/13C isotope fractionation resulting from photochemistry and escape. Both the 283
results of Mandt et al. (2012) and Nixon et al. (2012) fall into the timeline suggested by 284
interior models (Tobie et al., 2006) which suggests that the methane atmosphere is the result 285
of an outgassing episode that occurred between 350 and 1350 Ma. On Titan, both simple 286
(methane, ethane and propane) and complex hydrocarbons precipitate out of the atmosphere 287
onto the surface. Measuring the isotopic ratios (14N/15N; 12C/13C; D/H; 16O/18O; 17O/16O) and 288
abundances of the simple alkanes (e.g., methane, ethane and propane) would constrain the 289
formation and evolution of the methane cycle on Titan. Further measurements of radiogenic 290
noble gases such as 40Ar and 22Ne, which are typically markers of volatile elements from 291
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Titan’s interior can constrain outgassing episodes. Detection of 40Ar and tentatively 22Ne in 292
the atmosphere has provided circumstantial evidence of water-rock interactions and methane 293
outgassing from the interior (Niemann et al., 2010; Tobie et al., 2012). Recent measurements 294
by ground-based observatories including measurements of CO and its carbon and oxygen 295
isotopologues accompanied by the first detection of 17O in Titan and indeed in the outer Solar 296
System by Atacama Large Millimeter/submillimeter Array (ALMA) can be followed up on in 297
more detail by in-situ spectroscopic measurements (Serigano et al., 2016). E2T would 298
measure the composition and isotopic ratios of Titan’s upper atmosphere to determine the age 299
of methane in the atmosphere and characterize outgassing history.
300
On Enceladus, Cassini measurements by INMS (Waite et al., 2006, 2009, 2017) and UVIS 301
(Hansen et al., 2006, 2008) showed that plume gas consists primarily of water vapour with a 302
few percent other volatiles. In addition to H2O, as the dominant species, INMS was able to 303
identify CO2 (0.3-0.8%), CH4 (0.1-0.3%), NH3 (0.4-1.3%) and H2 (0.4-1.4%), in the vapour 304
plume as well as an unidentified species with a mass-to-charge (m/z) ratio of 28, which is 305
thought to be either N2 or C2H4, a combination of these compounds, or CO. The low mass 306
resolution of Cassini INMS is insufficient to separate these species, and the UVIS 307
measurements can only provide upper limits on N2 and CO abundance. Determining the 308
abundance ratio between these different species is, however, essential to constrain the origin 309
of volatiles on Enceladus and to assess if they were internally reprocessed. A high CO/N2 310
ratio, for instance, would suggest a cometary-like source with only a moderate modification 311
of the volatile inventory, whereas a low CO/N2 ratio would indicate significant internal 312
reprocessing.
313
In addition to these main volatile species, the possible presence of trace quantities of C2H2, 314
C3H8, methanol, formaldehyde and hydrogen sulphide has been detected within the INMS 315
data recorded during some Cassini flybys (Waite et al., 2009). Organic species above the 316
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INMS mass range of 99 u are also present but could not be further constrained (Waite et al., 317
2009). The identification and the quantification of the abundances of these trace species 318
remains very uncertain due to the limitations of the mass spectrometer onboard Cassini.
319
Except for the measurement of D/H in H2O on Enceladus (which has large uncertainty, 320
Waite et al., 2009), no information is yet available for the isotopic ratios in Enceladus’ plume 321
gas. The E2T mission would determine the isotopic ratios (D/H, 12C/13C, 16O/18O, 14N/15N) in 322
major gases compounds of Enceladus’ plume, as well as 12C/13C in organics contained in icy 323
grains. Comparison of gas isotopic ratios (e.g., D/H in H2O and CH4, 12C/13C in CH4, CO2, 324
and CO; 16O/18O in H2O, CO2, CO; 14N/15N in NH3 and N2) and with Solar System standards 325
would provide essential constraints on the origin of volatiles and how they may have been 326
internally reprocessed. Simultaneous precise determination of isotopic ratios in N, H, C and 327
O- bearing species in Enceladus’ plume and Titan's atmosphere would permit a better 328
determination of the initial reference values and a quantification of the fractionation due to 329
internal and atmospheric processes on both moons.
330
Noble gases also provide essential information on how volatiles were delivered to 331
Enceladus and if significant exchanges between the rock phase and water-ice phase occurred 332
during Enceladus’ evolution. The E2T mission would be able to determine the abundance of 333
40Ar, expected to be the most abundant isotope, as well as the primordial (non-radiogenic) 334
argon isotopes, 36Ar and 38Ar. The detection and quantification of 36Ar and 38Ar would place 335
fundamental constraints on the volatile delivery in the Saturn system. A low 36Ar/N2 ratio, for 336
instance, would indicate that N2 on Enceladus is not primordial, like on Titan (Niemann et al., 337
2010), and that the fraction of argon brought by cometary materials on Enceladus is rather 338
low. In addition to argon, if Ne, Kr, and Xe are present in detectable amounts, E2T would be 339
able to test whether primordial noble gases on Enceladus were primarily brought by a 340
chondritic phase or cometary ice phase, which has implications for all the other primordial 341
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volatiles. The 40Ar/38Ar/36Ar as well as 20N/21Ne/22Ne ratios would also allow for testing of 342
how noble gases were extracted from the rocky core. Abundance ratios between Ar/Kr and 343
Ar/Xe, if Kr and Xe are above detection limits, would offer an opportunity to test the 344
influence of clathrate storage and decomposition in volatile exchanges through Enceladus’s 345
ice shell.
346
The origin of methane detected in Enceladus’ plume is still uncertain. Methane, ubiquitous 347
in the interstellar medium, was most likely embedded in the protosolar nebula gas. The inflow 348
of protosolar nebular gas into the Saturn subnebula may have trapped methane in clathrates 349
that were embedded in the planetesimals of Enceladus during their formation. Alternatively, 350
methane may have been produced via hydrothermal reactions in Enceladus’ interior; a 351
possibility made more evident by the recent discovery of molecular hydrogen in Enceladus’
352
plume (Waite et al., 2017). Mousis et al. (2009) suggests that if the methane of Enceladus 353
originates from the solar nebula, then Xe/H2O and Kr/H2O ratios are predicted to be equal to 354
∼7×10−7 and 7×10−6 in the satellite’s interior, respectively. On the other hand, if the methane 355
of Enceladus results from hydrothermal reactions, then Kr/H2O should not exceed ∼10−10 and 356
Xe/H2O should range between ∼1×10−7 and 7×10−7 in the satellite’s interior. The E2T mission 357
by performing in situ analysis with high-resolution mass spectrometry of both the vapour and 358
solid phases would quantify the abundance ratios between the different volatile species 359
present in the plume of Enceladus, the isotopic ratios in major species, and the noble gas 360
abundance.
361
362
3.1.2 Sources and Compositional Variability of Enceladus’ Plume 363
The detection of salty ice grains (Postberg et al., 2009, 2011), the high solid/vapour ratio 364
(Porco et al., 2006; Ingersoll and Ewald, 2011), and the observations of large particles in the 365
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lower part of the plume (Hedman et al., 2009) all indicate that the plume of Enceladus 366
originates from a liquid source likely from the subsurface ocean rather than from active 367
melting within the outer ice shell (Figure 3). However, the abundance of the major gas species 368
observed by Cassini suggests some contribution from the surrounding cold icy crust should 369
also be considered. If plume gases exclusively originate from a liquid water reservoir, low- 370
solubility species would be more abundant than high-solubility compounds, which is not 371
apparent in the INMS data. The saltiness of the ice grains and the recent detections of 372
nanometer sized silica dust in E-ring particles, which are believed to come from Enceladus 373
(Hsu et al., 2011, 2015), and molecular hydrogen in Enceladus’ plume (Waite et al., 2017) all 374
indicate their origin is a location where alkaline high temperature hydrothermal reactions and 375
likely water-rock interactions are occurring.
376
FIGURE 3 377
Although the Cassini (Cosmic Dust Analyzer) CDA has constrained knowledge of plume 378
compositional stratigraphy, measurements of the absolute abundance and composition of 379
organics, silicates and salts are poorly constrained given the low spatial resolution (10 km), 380
low mass resolution and limited mass range of the CDA. The E2T ENIJA is capable of 381
providing a spatial accuracy of better than 100 m, allowing for a precise determination of 382
compositional profiles along the spacecraft trajectory (Srama et al., 2015). The Cassini INMS 383
provided only plume integrated spectra and is not able to separate gas species with the same 384
nominal mass. E2T INMS’ high mass resolution, which is 50 times larger than that of Cassini 385
INMS, would allow for separation of isobaric interferences, for example separating 13C and 386
12CH and CO and N2. Determining high-resolution spatial variations in composition is crucial 387
to establish whether the jets are fed by a common liquid reservoir or if jet sources are 388
disconnected, and if the local liquid sources interact with a heterogeneous region in the icy 389
crust. Variations in composition between the solid and gas phases as a function of distance 390
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from jet sources can also provide information about how the less volatile species condense on 391
the grains, thus constraining the eruption mechanisms. The E2T mission would allow for the 392
determination of the compositional distribution between both solid and vapour phases of 393
Enceladus’ plume, thus providing crucial constraints on the nature and composition of the jet 394
sources, and on the relative contributions of subsurface liquid reservoirs and the surrounding 395
cold icy crust. Spatial variations in composition within the plume and possible correlations 396
with the jet sources would permit for testing if the volatile compounds originate from a 397
common reservoir and how the less volatile compounds are integrated in the solid particles 398
during the eruption processes.
399
400
3.1.3 Geological Constraints on Titan’s Methane Cycle and Surface Evolution 401
There is an open question whether Titan’s methane-rich atmosphere is being actively 402
replenished, or if methane is being lost and Titan’s methane may eventually be depleted 403
(Yung et al., 1984). Cryovolcanism has been suggested as a mechanism by which methane 404
and argon can be transported from Titan’s interior to its surface (e.g., Lopes et al., 2013).
405
Cryovolcanic activity may also promote methane outgassing (Tobie et al., 2006); while 406
methane clathrates are stable in Titan’s ice shell in the absence of destabilizing thermal 407
perturbations and/or pressure variation, variations in the thermal structure of Titan’s outer ice 408
shell during its evolution could have produced thermal destabilization of methane clathrates 409
generating outgassing events from the interior to the atmosphere (Tobie et al., 2006; see also 410
Davies et al., 2016). A number of candidate cryovolcanic features have been identified in 411
Cassini observations (Lopes et al., 2013). E2T TIGER high-resolution colour images would 412
provide the data needed to determine the genesis of these features. Stratigraphic relationships 413
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and crater counting would provide a means by which the relative ages of these features may 414
be constrained.
415
A related question to the age of Titan’s atmosphere is if Titan’s climate is changing. At 416
present, most of the observed liquid methane is located in the north polar region (Aharonson 417
et al., 2009). There have been suggestions, however, that organic seas may have existed in 418
Titan’s tropics (Moore and Howard, 2010; MacKenzie et al., 2014), and/or in broad 419
depressions in the south (Aharonson et al., 2009; Hayes et al., 2011). Observations and 420
models suggest Titan’s methane distribution varies on seasonal timescales (e.g., Waite et al., 421
2007; Hayes et al., 2010; Turtle et al., 2011; Coustenis et al., 2013, 2016) or Milankovitch 422
timescales (Aharonson et al., 2009). Alternative models suggest that methane is being 423
depleted and Titan’s atmosphere is drying out (Moore and Howard, 2010). High-resolution 424
images of the margins and interiors of these basins would allow us to determine if they once 425
held seas. Identification of impact features or aeolian processes within these basins would 426
help to constrain the timing of their desiccation.
427
In addition to their inherent scientific interest, Titan's dunes serve as sensitive indicators of 428
climatic evolution (Lorenz et al., 2006; Radebaugh et al., 2008). Larger dune forms take 429
longer to form than smaller dune forms. In Earth's Namib desert, these differing timescales 430
result in large, longitudinal dunes that adhere to the overall wind conditions from the 431
Pleistocene 20,000 years ago, while smaller superposing dunes (sometimes called rake dunes, 432
or flanking dunes) have responded to the winds during our current interglacial and orient ages 433
accordingly (Lancaster et al., 1989). On Titan, E2T TIGER’s superior spatial resolution would 434
resolve these potential smaller dunes on top of the known longitudinal dunes, and would 435
therefore reveal if Titan's recent climate has been stable or if it has changed over the past few 436
Ma. The E2T mission would provide high-resolution colour imaging of Titan that can be used 437
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to characterize candidate cryovolcanic features that could be replenishing Titan’s atmosphere 438
and paleo-seas, or dune patterns that evidence changes in Titan’s climate.
439
Titan’s geology is unique in that liquid and solid organics likely play key roles in many of 440
the observed processes. These processes, in turn, play an important role in modifying these 441
organics, both physically and chemically. Understanding these modification processes is 442
crucial to investigating the complex chemistry occurring on this moon. Furthermore, study of 443
Titan’s geology allows us to investigate processes that are common on Earth, but in 444
drastically different environmental conditions, providing a unique way to gain insight into the 445
processes that shaped the Earth and pre-Noachian Mars.
446
Observations of Titan suggest the landscape is significantly modified by liquid organics 447
(e.g., Tomasko et al., 2005; Soderblom et al., 2007; Burr et al., 2013). Fluvial erosion is 448
observed at all latitudes, with a variety of morphologies suggesting a range of controls and 449
fluvial processes (Burr et al., 2013). High-resolution color imaging would provide insight into 450
the nature of this erosion: whether it is predominantly pluvial or sapping in nature and 451
whether it is dominated by mechanical erosion or dissolution. Dissolution processes are also 452
suspected to control the landscape of Titan’s labyrinth terrains (Cornet et al., 2015) and may 453
be responsible for the formation of the polar sharp edged depressions (Hayes et al., 2008):
454
E2T imaging would allow direct testing of these hypotheses.
455
Both fluvial and aeolian processes likely produce and transport sediments on Titan. Dunes 456
are observed across Titan’s equator (e.g., Radebaugh et al., 2008; Malaska et al., 2016) while 457
a variety of fluvial sediment deposits have been identified across Titan (e.g., Burr et al., 2013;
458
Birch et al., 2016). Detailed images of the margins of the dune fields would allow us to 459
determine the source and fate of sands on Titan. E2T images would also help determine 460
whether the observed fluvial features are river valleys or channels (cf. Burr et al., 2013) 461
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providing key information in obtaining accurate discharge estimates needed to model 462
sediment transport (Burr et al., 2006). E2T observation would provide insight into the primary 463
erosion processes acting on crater rims, which likely comprise a mixture of organics and 464
water ice (cf. Neish et al., 2015, 2016). Finally, E2T images may provide insight into the 465
nature of erosion that exists in Titan’s mid-latitudes, a region that shows little variability in 466
Cassini observations.
467
Of great interest in understanding the evolution of Titan’s surface is determining the nature 468
of the observed geologic units, including their mechanical and chemical properties. Fluvial 469
processes, the degree to which mechanical versus dissolution dominates and the existence of 470
sapping, reflect the material properties of the surface and therefore can be used as a powerful 471
tool to investigate the properties of the surface. E2T imaging would also allow us to 472
investigate the strength of the surface materials by constraining the maximum slopes 473
supported by different geologic units. Detailed color and stereo imaging of the boundaries of 474
units would also allow investigation of the morphology, topography, and spectral relationship 475
across unit boundaries. E2T would take high-resolution color images of Titan that would 476
elucidate the nature of the geological evolution of Titan’s organic-rich surface.
477
478
3.2 Habitability and Potential for Life in Ocean Worlds, Enceladus and Titan 479
3.2.1 Evidence for Prebiotic and Biotic Chemical Processes on Titan and Enceladus 480
Unlike the other ocean worlds in the Solar System, Titan has a substantial atmosphere, 481
consisting of approximately 95% nitrogen and 5% methane with trace quantities of 482
hydrocarbons such as ethane, acetylene, and diacetylene, and nitriles, including hydrogen 483
cyanide (HCN) and cyanogen (C2N2). Somewhat more complex molecules such as 484
cyanoacetylene, vinyl and ethylcyanide follow from these simpler units. In Titan’s upper 485
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atmosphere, Cassini has detected large organic molecules with high molecular masses over 486
100 u. In-situ measurements by the Cassini Plasma Spectrometer (CAPS) detected heavy 487
positive ions (cations) up to 400 u (Crary et al., 2009) and heavy negative ions (anions) with 488
masses up to 10,000 u (Coates et al., 2007) in Titan’s ionosphere. Whereas Cassini INMS 489
only had the ability to detect cations, E2T INMS can detect both cations and anions and can 490
do so with much better mass resolution than Cassini-INMS (and a fortiori than Cassini- 491
CAPS). It is thought that these heavy negative ions, along with other heavy molecules found 492
in the upper atmosphere, are likely the precursors of aerosols that make up Titan’s signature 493
orange haze, possibly even precipitating to the surface. While the compositions of these 494
molecules are still unknown, their presence suggest a complex atmosphere that could hold the 495
precursors for biological molecules such as those found on Earth. The ability to detect 496
prebiotic molecules in Titan’s atmosphere is currently limited by the mass range of the 497
Cassini INMS to the two smallest biological amino acids, glycine (75 u) and alanine (89 u), 498
and the limited mass resolution precludes any firm identification. While Cassini INMS has 499
not detected 75 or 89 u molecules, it has detected positive ions at masses of 76 u and 90 u, 500
which are consistent with protonated glycine and alanine, respectively (Vuitton et al., 2007;
501
Hörst et al., 2012). Experimental results from a Titan atmosphere simulation experiment 502
found 18 molecules that could correspond to amino acids and nucleotide bases (Hörst et al., 503
2012). The E2T mission would use high-resolution mass spectrometry to measure heavy 504
neutral and ionic constituents up to 1000 u, and the elemental chemistry of low-mass organic 505
macromolecules and aerosols in Titan’s upper atmosphere, and would monitor neutral-ionic 506
chemical coupling processes.
507
The plume emanating from Enceladus’ south pole probably contains the most accessible 508
samples from an extra-terrestrial liquid water environment in the Solar System. The plume is 509
mainly composed of water vapour and trace amounts of other gases (Waite et al., 2017). In 510
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addition, higher molecular weight compounds with masses exceeding 100 u, were detected in 511
the plume emissions (Waite et al., 2009; Postberg et al., 2015). The presence of CO2, CH4 and 512
H2 can constrain the oxidation state of Enceladus’ hydrothermal system during its evolution.
513
The minor gas constituents in the plume are indicative of high-temperature oxidation- 514
reduction (redox) reactions in Enceladus’ interior possibly a result of decay of short-lived 515
radionucleides (Schubert et al., 2007). In addition, H2 production and escape may be a result 516
of redox reactions indicative of possible methanogenesis similar to the process occurring in 517
terrestrial submarine hydrothermal vents (McKay et al., 2008; Waite et al., 2017). Further, the 518
high temperatures and H2 escape may have led to the oxidation of NH3 to N2 (Glein et al., 519
2008). Detection and inventory of reduced and oxidized species in the plume material (e.g., 520
NH3/N2 ratio, H2 abundance, reduced versus oxidized organic species) can constrain the redox 521
state and evolution of Enceladus’ hydrothermal system.
522
Cassini CDA measurements identified three types of grains in the plume and Saturn’s E-ring.
523
Type I and Type II grains are both salt-poor (Figure 4). Type I ice grains are nearly pure- 524
water ice while Type II grains also possess silicates and organic compounds and Type III 525
grains are salt-rich (0.5–2.0% by mass) (Postberg et al., 2009, 2011). The salinity of these 526
particles, the high solid/vapor ratio (Porco et al., 2006, Ingersoll and Ewald, 2011) of the 527
plume and observations of large particles in the lower part of the plume (Hedman et al., 2009) 528
indicate that the plume originates from a subsurface liquid source. The Cassini CDA later 529
detected nanometer sized silica dust particles in E-ring stream particles (Hsu et al., 2011, 530
2015) which indicate that water-rock interactions are likely taking place within this liquid 531
reservoir. Gravity and libration data demonstrated that this liquid reservoir was a subsurface 532
global ocean (Iess et al., 2014; Thomas et al., 2015) 533
FIGURE 4 534
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Hsu et al. (2015) suggest that the ocean should be convective in order to have silica 535
nanoparticles transported from hydrothermal sites at the rocky core up to the surface of the 536
ocean where they can be incorporated into icy plume grains (Hsu et al., 2015). To confirm 537
this hypothesis of current hydrothermal activity on Enceladus, a direct detection of silica and 538
other minerals within ejected ice grains is required. SiO2 nano-particles detected in Saturn’s 539
E-ring could be much better investigated and quantified by E2T ENIJA given its high dynamic 540
range (106–108). By performing high-resolution mass spectrometry of ice grains in Enceladus’
541
plume, the E2T mission would characterize the composition and abundance of organics, salts 542
and other minerals embedded in ice grains, as messengers of rock/water interactions. It would 543
also search for signatures of on-going hydrothermal activities from possible detection of 544
native He and further constrain recent measurements of native H2 found in Enceladus’ plume 545
(Waite et al., 2017).
546
547
3.2.2 Physical Dynamics in Enceladus’ Plume and Links to Subsurface Reservoirs 548
The total heat emission at Enceladus’ tiger stripes is at least 5 GW - possibly up to 15 GW, 549
(Howett et al., 2011), and in some of the hot spots where jets emanate, the surface 550
temperatures are as high as 200 K (Goguen et al., 2013). Cassini observations show that the 551
plume is made up of approximately 100 discrete collimated jets as well as a diffuse 552
distributed component (Hansen et al., 2008, 2011; Postberg et al., 2011; Porco et al., 2014).
553
The majority of plume material can be found in the distributed diffuse portion of the plume, 554
which likely originates from elongated fissures along Enceladus’ tiger stripes while only a 555
small portion of gas and grains are emitted from the jets (Hansen et al., 2011; Postberg et al., 556
2011). CDA measurements demonstrate that the majority of salt-poor grains tend to be ejected 557
through the jets and at faster speeds while larger salt-rich grains tend to be ejected more 558
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slowly through the distributed portion of the plume (Postberg et al., 2011). The ice to vapour 559
ratio can constrain how Enceladus’ plume material is formed and transported to the surface.
560
For example, ice/vapour ratios > 0.1–0.2 would exclude plume generation mechanisms that 561
require a large amount of ice grains to be condensed from vapour (Porco et al., 2006;
562
Ingersoll and Pankine, 2011). However, this ratio is poorly constrained with estimates ranging 563
from 0.05 (Schmidt et al., 2008) to 0.4 (Porco et al., 2006) to 0.35–0.7 (Ingersoll and Ewald, 564
2011). E2T high-resolution IR images and ENIJA can help constrain this important ratio.
565
Cassini ISS images used to track plume brightness variation, which is proportional to the 566
amount of grains in the plume, with the orbital position of Enceladus found more ice grains 567
are emitted when Enceladus is near its farthest point from Saturn (apocenter). It is not 568
understood if the plume vapour has such a variation. This temporal variation of the plume 569
indicates that it is tidally driven but could also be due to possible physical libration (Hurford 570
et al., 2009; Kite et al., 2016). Most recently, Kite et al. (2016) has suggested that the tiger 571
stripe fissures are interspersed with vertical pipe-like tubes with wide spacing that extend 572
from the surface to the subsurface water. This mechanism allows tidal forces to turn water 573
motion into heat, generating enough power to produce eruptions in a sustained manner.
574
TIGER can provide high spatial resolution thermal emissions maps to constrain the amount of 575
energy dissipated between the tiger stripes. The E2T mission would use high resolution IR 576
imaging of the south polar terrain and mass spectra of the grains to provide new details of its 577
surface and constrain the links between plume activity, subsurface reservoirs and deep 578
hydrothermal processes.
579
580
581
582
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3.2.3 Geological Evidence for Interior-Surface Communication on Titan 583
Geological processes such as tectonism and cryovolcanism indicate communication between 584
the surface and subsurface. While Titan’s surface offers a wealth of geological processes, the 585
Cassini data lack the resolution needed in which to constrain the detailed nature of these 586
processes, and thus to understand the extent that Titan’s surface may be chemically 587
interacting with its water-rich interior. Also of great importance to habitability are the 588
transient H2O melt sheets and flows (e.g., Soderblom et al., 2010) associated with impacts. On 589
Titan, several features with volcanic landforms, lengthy flows, tall mountains, large caldera- 590
like depressions, have been identified as possible cryovolcanic sites but could also possibly be 591
due to other endogenic processes (Lopes et al., 2016; Solomonidou et al., 2016). At present, 592
the Hotei Regio flows and the Sotra Patera region, which includes Sotra Patera, an elliptical 593
deep depression on Titan, Mohini Fluctus, a lengthy flow feature, and Doom and Erebor 594
Montes, two volcanic edifices, are considered to host the strongest candidates for 595
cyrovolcanism on Titan (Lopes et al., 2013; Solomonidou et al., 2014, 2016).
596
A variety of mountainous topography has been observed on Titan(Radebaugh et al., 2007;
597
Cook-Hallett et al., 2015). The observed morphologies of many of Titan’s mountains suggest 598
contractional tectonism (Mitri et al., 2010; Liu et al., 2016). This is somewhat surprising, 599
however, since tectonic landforms observed on other ocean worlds and icy satellites in the 600
outer solar system appear to be extensional in nature. Understanding Titan’s tectonic regime 601
would, thus not only provide insight into the transport of material between surface and the 602
interior, but also into the evolution of the other ocean worlds. We would test the hypothesis 603
that Titan’s mountains are formed by contraction by mapping the faults driving mountain 604
formation in topographic context. The shape of the fault outcrop draped against topography 605
would allow us to measure the dips of faults, which would be ~30 degrees to the horizontal 606
for compressive mountains and ~60 degrees for extensional mountains.
607
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In addition to cryovolcanism and tectonism, which may transport water to Titan’s surface, 608
impact craters likely have created transient liquid-water environments on Titan’s surface.
609
Because of Titan’s dense atmosphere, models suggest that melt sheets and flows associated 610
with impact craters may remain liquid for 104–106 years (Thompson and Sagan, 1992;
611
Artemieva and Lunine, 2005), though the stability of such melts is questioned (Senft and 612
Stewart, 2011; Zahnle et al., 2014) and detailed imaging of the floors of young craters is 613
needed to constrain these models. Titan offers numerous pathways for interaction between its 614
organic-rich surface and liquid water. E2T would provide high-resolution mapping (30 615
m/pixel with DTM vertical resolution of 10 m) that would offer the ability to distinguish 616
cryovolcanic features and to investigate the morphology of Titan’s mountains and impact 617
craters.
618
619
4. Scientific Payload 620
The Explorer of Enceladus and Titan (E2T) has a focused payload that would provide in-situ 621
mass spectrometry and high-resolution imaging of Enceladus’ south polar terrain and plume, 622
and Titan’s upper atmosphere and surface, from a solar-electric powered spacecraft in orbit 623
around Saturn. The in-situ measurements of Titan’s upper atmosphere would be acquired 624
during 17 flybys with an altitude as low as 900 km. At Enceladus, in-situ measurements 625
would be conducted during 6 flythroughs of the plume and flybys of the south polar terrain at 626
altitudes between 50 and 150 km. At the closest approach the velocity of the S/C with respect 627
to Enceladus surface is ~5 km/s and with respect to Titan surface is ~7 km/s. Imaging data 628
will be collected during inbound and outbound segments of each flyby.
629
The E2T mission model payload consists of three science instruments: two time-of-flight 630
mass spectrometers, the Ion and Neutral gas Mass Spectrometer (INMS) and the Enceladus 631
